The development of modern rubber materials has made possible the manufacture of a wide range of elastomeric articles having varying properties of strength and chemical resistance. As synthetic latex materials have developed, various elastic and polymeric materials have been adapted for use in making a variety of articles of manufacture. One useful class of synthetic rubber material compounds includes the nitrile rubber class, which is widely used to make articles such as gloves and oil resistant seals.
Elastomeric articles requiring the highest elongation and greatest ease to stretch, such as surgical or examination gloves, balloons, and condoms have traditionally been made from natural rubber latex. While nitrile rubber products are typically more difficult to stretch, one of the advantages of nitrile rubber over natural rubber latex substrates is that nitrile rubber products do not contain natural latex proteins which can become a significant allergy issue for some users. Other advantages of nitrile materials over natural rubber latex include much better chemical resistance, especially to fatty and oily substances, and better puncture resistance. Hence, nitrile-rubber-based gloves have become desirable as a substitute for natural rubber products.
While hospitals, laboratories, or other work environments that may use rubber gloves often want to go “latex free” to better protect their workers, the normally higher cost of nitrile products often limits their ability to make the change. Another hindrance toward making the change is that nitrile gloves traditionally have been stiffer, hence are much less comfortable to wear as compared to similar types of gloves made from natural rubber latex materials. For instance, natural rubber latex (NRL) examination gloves typically require a stress of about 2.5 MPa (58 psi) to stretch to an elongation of about 300% over original dimensions. This often is referred to as the glove's 300% modulus. Nitrile exam gloves, on the other hand, typically require more than twice that amount of stress (˜5 MPa, 116 psi) to achieve the same 300% strain. While vinyl can be another synthetic choice, vinyl is often seen as a lower performance choice.
Currently, no synthetic latex examination gloves are available on the commercial market that exhibit force-strain properties that are close to that of natural rubber latex gloves, not to mention being either similar or the same as natural rubber-based gloves in these terms. Force-strain properties refer to a direct measurement of how a material responds (stretches) in response to an applied force, regardless of the thickness of the material. Stress-strain properties in contrast measure the response to an applied force per unit cross sectional area of the material.
Nitrile rubber, a synthetic polymer often used in emulsion (latex) form to manufacture medical and industrial gloves is a random terpolymer of acrylonitrile, butadiene, and a carboxylic acid such as methacrylic acid. It can be crosslinked by two separate mechanisms to improve its strength and chemical resistance. The first mechanism of crosslinking occurs by ionically bonding carboxylic acid groups together using multivalent metal ions. These ions are typically supplied through addition of zinc oxide to the emulsion. Normally the strength and stiffness/softness of the polymer is very sensitive to this type of crosslinking. The other crosslinking mechanism is a covalent crosslinking of the butadiene segments of the polymer using sulfur and catalysts known as rubber accelerators. This covalent crosslinking is especially important for development of chemical resistance. Gloves are often formed by first placing a coagulant solution, often calcium nitrate on ceramic glove moulds, then dipping into the nitrile latex to cause local gelation of nitrile rubber over the mould surface.
Several prior approaches to softening nitrile rubber articles involved strongly limiting or completely omitting zinc oxide and other materials capable of ionically crosslinking carboxylated nitrile rubber, such as those described in U.S. Pat. Nos. 6,031,042 and 6,451,893. In addition to not yielding force-strain properties similar to those of comparable natural rubber products, this method can result in a material having lower strength, the need for higher curing temperatures, the need for extraordinarily high levels of other chemicals that may cause skin irritation, or it may lead to processing difficulties such as thickening of the nitrile latex before dipping.
Other approaches to making a nitrile glove more comfortable, such as those described in U.S. Pat. Nos. 5,014,362 and 6,566,435, have relied on stress relaxation over time and require constantly applied levels of strain to cause that relaxation or softening. Such determination measures are difficult to maintain and can be unrealistic in real world practice and use.
A need exists for a nitrile-based polymer article that can successfully combine the benefits of nitrile materials with the greater pliability or softness of natural rubber latex without the need to apply conditions required for softening caused by stress relaxation. There is a need for a kind of nitrile glove that can incorporate a polymer formulation and product dimensions to simulate the comfort and softness associated with natural rubber latex products, while simultaneously maintaining the protective and non-allergenic properties of nitrile rubber. The glove, when worn, still enables the elastomeric material to exhibit physical strain or stress profiles similar to those of natural rubber, without exposure to natural rubber's associated problems.